In the prior art, there is known a vehicular passenger protecting device for expanding an air bag with gases generated by a gas generator so as to ensure the safety of the passenger in the event of a vehicular collision. This gas generator is housed, for example, in a gas generator casing together with an ignition device in which a gas generation agent has an ignition agent. To satisfy light weight specifications, the gas generator casing is usually made of a light metal or material such as an aluminum alloy. This material has its mechanical strength lowered when overheated to a high temperature. Therefore, the gas generator made of such material will not encounter problems in the ordinary case, in which the ignition agent is ignited at the collision by the ignition device and in which the gas generation agent is then ignited to generate the gases. In case, however, the vehicle is extraordinarily overheated from the outside as by fire or during disposal the ignition agent or the gas generation agent is ignited after the gas generator casing has lost its mechanical strength, if its ignition temperature is higher than the temperature at which the mechanical strength of the gas generator casing drops. Then, this gas generator casing may be broken and its pieces scattered by the pressure built up in the gas generator.
One solution to this problem is disclosed in U.S. Pat. No. 4,561,675, for example. The disclosure is directed to an auto ignition device which is constructed such that there is confined in a container (i.e., a metal casing) made of a metal foil and acting as an independent member a primary ignition agent capable of spontaneous ignition at a temperature of about 177.degree. C., low enough for retaining the mechanical strength of the container, such that this container is fixed within the gas generator casing by means of a refractory adhesive or cushion, and such that the primary ignition agent is directed toward a secondary ignition agent or a gas generation agent ignitable at about 343.degree. C.
However, the gas generator casing and the container of the '675 patent are independent from each other and are mechanically fixed by means of the refractory adhesive or cushion. When the gas generator is heated from the outside by a flame or the like, the heat is transferred through the gas generation agent casing, the refractory adhesive or cushion and the container to the primary ignition agent. Thus, the structure is defective in terms of poor thermal conductivity. If, moreover, a clearance should be established between the gas generator casing and the container, the thermal conduction is deteriorated even more, to lengthen the time period for igniting the primary ignition agent. As a result, the primary ignition agent is ignited, after the temperature for lowering the mechanical strength of the gas generator casing has been reached, so that the gas generator casing might be broken by internal pressure causing scattering of its pieces.
Normally, smokeless powder is used for the automatic ignition agent, but smokeless powder generates decomposition gas when maintained at elevated temperatures for a long period. For example, according to U.S. Pat. No. 4,561,675, where an independent container member comprising a metal foil contains the automatic ignition agent, the decomposition gas can build up sufficient inner pressure to destroy the container itself.
In U.S. Pat. No. 4,944,528 on the other hand, there is disclosed technology concerning an auto ignition device. This auto ignition device is constructed such that a primary ignition agent casing confining a primary ignition agent ignitable at a surrounding temperature of about 160.degree. C. to 180.degree. C. is fitted through an insulator in the opening of a gas generation agent casing.
The primary ignition agent casing is fitted in the gas generator casing through a molded member and a plate-shaped insulator. These members have their boundary gas-tightness lowered in the temperature cycle in the natural circumstances due to the difference in the thermal expansions of the individual members. This causes the secondary ignition agent to absorb the moisture, thus inviting a defect that the gas generator may be inoperative, even if necessary.
It is, therefore, an object of the present invention to provide an auto ignition device in a gas generator for a vehicular passenger protecting device, which is able to enhance the heat sensitivity to heat from the outside and to ignite the ignition agent or the gas ignition agent safely and reliably while preventing moisture absorptivity.